LEC 19: GAS EXCHANGE

• Atmospheric air vs Alveolar air

  • Atmospheric air:
  • Higher oxygen concentration
  • Lower carbon dioxide concentration
  • Lower humidity
  • Facilitates oxygen absorption in the bloodstream
  • Essential for efficient gas exchange in the lungs
  • Alveolar air:
  • Lower oxygen concentration
  • Higher carbon dioxide concentration
  • Higher humidity
  • Facilitates CO2 elimination from the body
  • Essential for efficient gas exchange in the lungs
• Dalton's Law


  • Total pressure exerted by a gas mixture = sum of partial pressure of individual gases
  • Pertinent to the composition of alveolar gas
  • Governs the exchange of gases between alveoli and blood during external respiration
• Henry's Law


  • Amount of gas dissolved in a liquid is proportional to its partial pressure and solubility
  • Dictates the exchange of gases between alveolar air and blood
  • Regulates the dissolution of gases in blood plasma during internal respiration
• How oxygen is transported in blood

  • 1.5% dissolved in plasma
  • 98.5% bound to hemoglobin in RBCs
• Hemoglobin's affinity for oxygen

  • Changes based on temperature, partial pressure of carbon dioxide (PCO2), blood pH, production of bisphosphoglyceric acid (BPG) from glycolysis
• Oxygen-hemoglobin dissociation curve

  • Demonstrates hemoglobin's affinity for oxygen
  • Relatively flat at high partial pressure of oxygen
  • Low PO2 = hemoglobin releases oxygen to tissue, especially during exercise
  • During vigorous exercise = more oxygen is unloaded to tissues despite high plasma PO2 levels
• Types of hypoxia

  • Anemic hypoxia
  • Ischemic hypoxia
  • Histotoxic hypoxia
  • Hypoxemic hypoxia
• Carbon monoxide poisoning


  • Caused by carbon monoxide binding to hemoglobin with greater affinity than oxygen
  • Reduces the blood's oxygen-carrying capacity
• How carbon dioxide is transported in the blood

  • Dissolved in plasma (7-10%)
  • Bound to hemoglobin (20%)
  • Bicarbonate ions (HCO3-) (majority)
• Conversion of CO2 to bicarbonate ions

  1. CO2 combines with water and dissociates, producing carbonic acid and bicarbonate ions that can release or absorb H+

  2. Chloride ions move into RBC to maintain charge balance ("chloride shift")
• PO2


  • Controls perfusion by changing arteriolar diameter
  • Optimizes perfusion and maximizes oxygen uptake into the blood
  • Low PO2 in muscles during exercise, hemoglobin easily releases oxygen to body tissues
  • High plasma partial pressures of oxygen, hemoglobins unloads little oxygen
  • During vigorous exercise, plasma partial pressure of oxygen falls dramatically so much more oxygen can be unloaded to the tissues
• PCO2


  • Controls ventilation by changing bronchiolar diameter
  • Increases ventilation so there can be more rapid release of CO2
• Carbonic acid-bicarbonate buffer system of the blood is formed when CO2 combines with water and dissociates, producing carbonic acid and bicarbonate ions that can release or absorb H+
• Slow, shallow breathing


  Causes an increase in CO2 in blood resulting in a drop in pH
• Rapid, deep breathing


  Causes a decrease in CO2 in blood, resulting a rise in pH